People starting saying, "I haven't lost any weight. This is pointless, I'm not accomplishing anything," said Hutchinson.

After a few months of increased exercise, they are healthier because they've reduced risk factors such as blood sugar levels. Even though a person may not be losing weight, his health has improved in ways that might not be measured.

"They're stuck in this paradigm that weight is the ultimate barometer for fitness. They don't realize the progress they've made and give up."

6) Low-intensity exercise burns more fat.

In general, low intensity exercise has its place -- it's less stressful on joints.

The myth is that if you exercise too intensely, you end up burning carbohydrates instead of fat.

It's the most dangerous type of myth because there's a kernel of truth in it, Hutchinson said.

The more intensely you exercise, the higher proportion of carbs you burn. You may burn less fat, but the total amount of calories burned is higher and that is the bigger picture.

When your body has burned up all the carbs, it starts burning fat.

"You can ignore zones and pay attention to how many calories you burn, which ultimately determines how much body fat you're going to lose," Fitzgerald said.

5) Chug a protein shake after workout.

"It's eating another meal," said Macdonald, a personal trainer who helped TV host Chelsea Handler get in shape.

Protein shakes, powders and bars are good for emergencies, but "they're the lowest quality food."

"You're better off eating real food," he said.

The products are more processed. The best way to get protein is through foods such as a turkey sandwich, Greek yogurt with nuts and fruit.

Martin Gibala, chairman of the department of kinesiology at McMaster University in Hamilton, Ontario, agrees. "Protein sources in real food are Number 1. Cheaper and real food may provide other benefits, vitamins and minerals. And some of the components in food may act synergistically in ways we don't understand."

"When we isolate the compound we think works, it's not as good as the real foods."

4) You can spot reduce for tight abs or toned arms.

You may have crunched in vain.

You won't see muscle definition or a nice six-pack despite how many crunches you do, because of the layer of fat resting on top of your muscles.

"Don't focus on a body part. Try to get them all," said Burron, a spokeswoman for the American Council on Exercise. "You might have beautiful triceps -- it may not be flopping all over the place. Until the fat is gone, most people wouldn't know it's there."

3) As long as I go to the gym 30-45 minutes, that gives me a pass to do what I want for rest of the day.

The gym doesn't negate a bad diet.

Also, emerging research suggests that if you're sedentary most of the day, it may not matter how hard or often you exercise.

People who spend more time sitting during their leisure time have an increased risk of death, regardless of daily exercise.

In a study of more than 123,000 healthy people, the American Cancer Society found that women who spent more than six hours a day sitting were 40 percent more likely to die sooner than women who sat less. Men who sat more had 20 percent increased risk of death.

"The ongoing perception is that people need to feel pain through the entirety of their workout or they're not getting the benefit -- that one's very frustrating to me," said Burron, a personal trainer. "You shouldn't be exercising at a level of pain ever."

Feeling discomfort during a workout is OK.

"If it's so intense you're thinking of passing out, you can't continue this session for longer, then it's too difficult and you're at increased risk for injuries or burnout," Burron said.

"You want to exercise smarter, not harder," she added. "That's the premise. You don't have to kill yourself. You just have to be smart about it."

1) Stretching will help prevent injuries.

A growing number of studies challenge the entrenched assumption that stretching helps prevent injuries.

"The way we were taught to stretch, to try to touch your toes -- there's little evidence it prevents injuries," Hutchinson said.

A review published in 2007 of 10 randomized studies about stretching after or before physical activity found that "muscle stretching does not reduce delayed-onset muscle soreness in young healthy adults."

Static stretching is when you stay in place, bend over to touch your toes, or try to pull your ankles towards your hips.

A study presented this year at the American Academy of Orthopaedic Surgeons found that such static stretching before a run neither prevents nor causes injury.

Then a study published this month in Medicine & Science in Sports & Exercise found that static stretches that last longer than a minute could be detrimental to performance.

Athletes often swing their arms and warm up before a game. That type of dynamic stretching such as high knee jogs, walking lunges can help move your muscles through different ranges of motions.

This type of dynamic stretching is different from clutching your limbs, because it focuses on movement.

A study published in the Journal of Strength & Conditioning Research found professional soccer players who practiced dynamic stretching had higher range of motion than when they practiced only static stretches.

While toe-touches and extra flexibility might be required in gymnastics or figure skating, it's irrelevant for more everyday activities like basketball or weightlifting.